Tumor necrosis factor (TNF) is a cytokine produced by a number of cell types, primarily by mononuclear phagocytes. At present, two different TNFs have been identified: TNF-.alpha. and TNF-.beta. (lymphotoxin). Both TNF-.alpha. and TNF-.beta. initiate their effects by binding to specific cell receptors.
TNF-.alpha. and TNF-.beta. (hereinafter called "TNF") are known to exert both beneficial as well as deleterious effects on a number of different target cells involved in the inflammatory response. Among its many effects, TNF, for example, stimulates the growth of fibroblasts and induces in these cells the synthesis of collagenase, prostaglandin E2 and IL-6. TNF also decreases in adipocytes the activity of lipoprotein lipase, activates osteoclasts and increases in endothelial cells adhesivity for blood leukocytes.
However, TNF has also extremely deleterious effects: over-production of TNF can play a major pathogenic role in several diseases, for example, TNF-.alpha. is known to be a major cause for the symptoms of septic shock. In some diseases, TNF may cause excessive loss of weight (cachexia) by suppressing activities of adipocytes and by causing anorexia (TNF-.alpha. was therefore called cachectin). See, e.g. Beutler et al., Annu. Rev. Biochem., 57, pp. 507-518 (1988) and Old, Sci. Am. 258, pp. 41-49 (1988). Excessive TNF production has also been demonstrated in patients with AIDS.
In order to counteract the cytotoxic effects of TNF, ways were sought to antagonize or eliminate endogenously formed or exogenously administered TNF. Furthermore, ways are being sought to induce specifically only some of the many effects of TNF or restrict its action to a specific kind of target cells. The first attempt in this direction was the development of monoclonal antibodies which neutralize the TNF-.alpha. cytotoxic activity. Such monoclonal antibodies are described in EP 186 833 and in Israel Patent No. 73883.
As stated above, TNF initiates its function by binding to specific cell surface receptors. Two such TNF receptors (hereinafter "TNF-R") which are expressed differentially in cells of different kinds are known, the p55-TNF receptor and the p75-TNF receptor (p55-TNF-R and p75-TNF-R). Two proteins called TBP-I and TBP-II which bind specifically to TNF have been shown to cross-react immunologically with the two receptors. Both proteins provide protection against the in vitro cytocidal effect of TNF and both bind TNF-.beta. less effectively than TNF-.alpha.. It was found that the formation of the TBPs occurs by proteolytic cleavage of the cell surface TNF-Rs, resulting in release of a major part of their extracellular domain (see EP 308 378, 398 327 and 444,900). Indeed, the sequences of the amino acids in TBP-I and TBP-II were found to be fully identical to sequences found in the extra-cellular domains of the cell-surface receptors, but do not contain any part of the intracellular domain of the receptors.
These findings imply that the inhibition of TNF function by TBP-I and TBP-II reflects the conservation, in TBP-I and TBP-II, of part of the structural features of the cell surface TNF-Rs, which are important for binding of TNF by the receptors and the initiation of cell response to TNF thereby. Due to this conservation of structure, TBP-I and TBP-II have the ability to compete with the cell surface TNF-Rs for TNF and thus block its function.
It is known that TNF, in its natural state, exists as a multimer (trimer) consisting of three identical polypeptide chains, each with a molecular size of about 17,000 D.
To elicit its effects, TNF must bind to the TNF Receptors in its trimeric form. Although the TNF monomer also binds to cells (but at a lower affinity when compared with the TNF trimer), it has no effect.